A pilot, whose flight instructor died after takeoff, thought he was "just pretending to take a nap," according to a report from the U.K.’s Air Accidents Investigation Branch. 

On June 29, the 57-year-old instructor and licensed commercial pilot suffered from "a sudden fatal heart attack" while flying with a qualified co-pilot in Lancashire, England. 

The duo had planned to fly a Piper PA-28-161 from Blackpool Airport to another airfield. 

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Shortly after takeoff, though, the pilot recalled seeing the instructor's head roll back. At first, the co-pilot thought the instructor was "just joking" and continued to fly, the report said. 

"The pilot knew the instructor well and thought he was just pretending to take a nap whilst the pilot flew the circuit, so he did not think anything was wrong at this stage," the read.

Eventually, the instructor "slumped over" and his head was resting on the pilot's shoulder as he continued to fly. 

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"The pilot still thought the instructor was just joking with him and continued to fly," the report said. 

After landing, the pilot knew something was wrong when the instructor was still resting on his shoulder and "not responding." 

Fire and air ambulance medical crews tried to revive him, but he "remained unresponsive," officials said. 

It was determined that the instructor died from acute cardiac failure. The report showed that the 57-year-old man had a medical history of high blood pressure and had been taking medication for it since 2002. 

However, he had a medical evaluation in February 2022, just four months prior to the incident. 

Additionally, people who spoke with the instructor the morning before the flight said he was "his normal cheerful self" and that there "were no indication that he was feeling unwell," the report read. 

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He even flew with three people for a trial lesson earlier that morning. 

The instructor was a full-time senior flight instructor that had accumulated a total of 8,876 flying hours. 

Officials plan to review the circumstances of the incident to determine if anything can be learned or if any changes should be made to current guidance, according to the report.